Thymic nurse cells (TNCs) are specialized epithelial components that contain T cells completely enclosed in intra-cytoplasmic vacuoles. Very little information about their function has been reported because of the lack of pure TNC populations in quantities necessary for thorough analyses. However, they are believed to play a role in thymocyte development. Recently, this laboratory has been able to immortalize TNCs with both wild type SV40 and its temperature sensitive mutant, tsA58. Surprisingly, cells from both wild type and ts TNC lines have been shown to internalize thymocytes in in vitro studies. Intact cells can be visualized in cytoplasmic vacuoles. These findings were published in the Journal of Cellular Immunology. This is the first report of the isolation of TNC lines able to internalization and maintain viable another type of cell in tissue culture. Since that time, we have shown that TNCs bind and internalize CD4+CD8+ thymocytes exclusively. These findings suggest a finding for TNC in the restriction process. It then was important to show that TNCs were actually internalizing immature thymocytes. We accomplished this using various microscopic techniques. The most exciting result was obtained using long term video microscopy. We were able to capture the internalization of a thymocyte by a TNC. This video has been shown in many seminars, and still photographs were published as a part of another publication. In that study, we proposed a model for the mechanism of internalization. A ts line has yielded much information as well. This line, tsTNC-1 has been used to show that TNCs have the ability to rescue a subset of double positives from cell death. As has been shown earlier by other investigators, the switch to the single positive phenotype does not occur in co-incubation experiments with TNCs. Earlier reports show that single positives do not appear until the thymocytes reach the medulla. Most excitingly, the rescue potential of TNC involves their production of oxytocin. The latter results are the subject of a manuscript in preparation. A subpopulation of CD4+CD8+ expressing thymocytes co-cultured with tsTNC-1 cells at 320C remained viable for over 4 days. Rescue of double positives was not detected at 380C. The rescued population of thymocytes expressed the protooncogene bcl-2 and showed a reduced level of apoptosis as measured by the DNA fragmentation assay. TNC rescue resulted in a shift of CD4+CD8+ thymocytes from immature TCRlow PNAr high cells to the more mature TCRint PNArlow phenotype. Survivors expressed high levels of both CD4 and CD8 on their cell surfaces but no changes in cell surface levels of HSA nor CD69 was detected. The addition of antibodies to either class I or class ll MHC antigens greatly reduced the rescue activity of tsTNC-1 cells at 32oC. These results suggest that TNCs rescue an early CD4+CD8+ subset through a TCR/MHC interaction. The resulting survivors mature to a TCRint PNArlowCD4highCD8high HSAhigh CD69- intermediate phase within the double positive stage of development. ;